The Bortle Scale

The Bortle scale is a nine-level numeric scale that measures the night sky’s brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies.

(Source: Wikipedia)

The stars circle around the North Star on a cold winter evening in Crater Lake National Park in Oregon. Michael Bonocore captured the movement of stars in the night sky — called star trails — by combining 400 separate images. 

Photo courtesy of Michael Bonocore.